Diamond particles, including diamond nanoparticles or nanodiamonds, have potential uses in a wide variety of applications including plating, lubricating oils, polishing, coating agents for metal surfaces, abrasives, and biomedical devices.
Nanodiamonds may be produced by the detonation of explosives with a negative oxygen balance in hermetic tanks. These detonation nanodiamonds have particle sizes typically in the range 1-50 nm and are particularly prone to agglomeration, forming tightly aggregated secondary particles or clusters.
Nanodiamonds and micron-sized diamond particles may also be synthesised by high temperature and high pressure (HTHP) methods. These diamond particles typically have particle sizes in the range 1 nm-1000 micron. Alternative sources of nano- or micron-sized diamond particles include diamond particles formed by HTHP methods and natural micron diamond powder. These diamond particles are also prone to aggregation forming aggregated secondary particles or clusters. Sizes of aggregated diamond particles vary widely but can be up to several 100s of microns in size.
A known method for separating these tightly aggregated nanodiamond clusters is via wet-stirred-media-milling in water using micrometer-sized beads to break up the aggregates and simultaneously dispersing them in water. This method has been used to form clear colloidal solutions of nanodiamonds (Adv. Mater. 2007, 19, 1201-1206). This method is only suitable for the formation of nanodiamond dispersions in solvents since the nanodiamonds tend to re-agglomerate after drying.
US2010/0298600 describes a method for the chemical functionalization of the surface of nanodiamonds in a liquid phase. The method uses a wet milling process similar to that described above, prior to treatment with acid. The method attempts to de-aggregate particles using an acid treatment step, which leads to formation of carboxylic acid groups on the surface of the nanodiamonds. There is no step in the process which actually breaks apart the clusters of nanodiamonds. In addition, the presence of carboxylic acid groups on the surface of the nanodiamonds will actually promote aggregation of the nanodiamonds after drying.
EP2535312 describes a method for dispersing nanodiamonds in solution. The method involves a wet milling process in the presence of an aqueous metal hydroxide and chemically binding a metal ion from the metal hydroxide solution with a reactive group present on the nanodiamond surface. For example a carboxylic acid group present on the nanodiamond surface may react with the metal hydroxide to form nanodiamond-metal salts as shown in Reaction 1 below, where ‘ND’ is used to represent the nanodiamond.ND-COOH+MOH→ND-COO-M++H2O  Reaction 1
By substituting a hydrogen atom with a metal ion the electrical repulsion between the nanodiamond particles is increased, improving dispersibility of the nanodiamonds in solution and larger ionic diameters lead to better dispersions. Although this method does involve a step in which a dry nanodiamond powder is obtained, the nanodiamonds do tend to re-aggregate after drying, but then re-disperse when mixed with a dispersion solvent. This method is also restricted to nanodiamond dispersions in which the diamond particles are functionalised with metals which could limit the end use of the nanodimond product.
It would be desirable to provide an improved method for the separation of secondary diamond particle clusters into discrete particles, providing diamond particles that are well dispersed in dry powder form, as well as in solution. In addition, it would be desirable to provide a method in which the surface functionalisation of the nanodiamond particles can be altered to suit the end use of the product.